Originally posted by Lowell Goudge the lens does 1:2 presently, and has a total extension in the focusing helix of 50mm,, when focused at infinity, the initial extension is obviously 100mm, To reach 1:1 on any lens you need a lens extension in total of 2 times focal length because 1:1 is when Image to lens distance = lens to focusing plane distance = 2x focal length.

therefore to get to 1:1, you need the lens 200mm from the focal plane. infinity = 100mm, focusing helix at min focus = 50mm, hence the correct answer is an additional 50mm of extension.

Most extension tubes come in 3 tube sets which add toi usually about 64mm,. therefore a 3 part set will give you a little better than 1:1.

In M42 the sets are shorter, and I an not sure in total it adds to 50mm, I would need to check

That's correct for magnification due to extension:

Magnification = (1+ extension/focal.length.)

*true for m42 lenses as well.*
Be careful though; newer IF lenses (Internal Focus) focus by shortening their focal length so the magnification due to extension may be greater than the equation above predicts.